THE   UNIVERSITY 

OF   ILLINOIS 

LIBRARY 


UNIVERSITY  OF  lUINOISUWRY 

NOV    1  1918 
UNIVERSITY  OF  ILLINOIS 

Agricultural  Experiment  Station 


BULLETIN  No.  182 


POTASSIUM  FROM  THE  SOIL 


BY  CYEIL  G.  HOPKINS   AND  J.  P.  AUMEB 


URBANA,  ILLINOIS,  MAY,  1915 


SUMMARY  OF  BULLETIN  No.  182 

1.  Potassium  can  be  liberated  as  needed  from  the  inexhaustible 
supply  naturally  contained  in  the  normal  soils  of  Illinois. 

2.  The  amount  of  potassium  taken  up  from  ordinary  Illinois  soil 
by  clover  is  from  two  to  three  times  the  amount  required  for  plant 
growth.     The  excess  probably  is  merely  tolerated,  as  is  sodium  and 
silicon,  both  of  which  are  present  in  the  soil  solution  and  are  taken  up 
by  plants  in  considerable  amounts,  altho  neither  is  essential  for  plant 
growth. 


POTASSIUM  FROM  THE  SOIL 

BY  CYRIL  G.  HOPKINS,  CHIEF  IN  AGRONOMY  AND  CHEMISTRY,  AND 
J.  P.  AUMEE,  ASSOCIATE  IN  CHEMISTRY 

Potash  is  so  widely  advertised  and  commercial  potassium  has  been 
given  such  a  prominent  place  in  most  of  the  experiments  relating  to 
soil  fertility,  both  in  this  country  and  abroad,  that  the  greatest  natural 
source  of  potassium, — the  soil, — like  the  inexhaustible  atmospheric 
supply  of  nitrogen,  is  not  generally  understood  or  appreciated. 

The  fact  is  that  the  total  amount  of  commercial  potassium  applied 
annually  to  all  the  farms  of  the  United  States  is  no  more  than  is  con- 
tained in  one  square  mile  of  common  corn-belt  land  to  a  depth  of  six 
feet,  and  the  important  potash  problem  is  how  to  liberate  it  as  needed 
from  the  inexhaustible  supply  already  contained  in  all  normal  soils. 

COMMERCIAL  POTASSIUM 

Potassium  (also  called  kalium — K)  is  an  important  plant-food  ele- 
ment. It  makes  up  83  percent  of  potash  (K,0),  about  42  percent  of 
commercial  potassium  chlorid  (KC1),  incorrectly  called  ''muriate  of 
potash,"  about  42  percent  of  commercial  potassium  sulfate  (K2SO4), 
and  about  10  percent  of  kainit,  a  crude  salt  taken  directly  from  the 
German  potash  mines. 

In  the  Journal  of  Industrial  and  Engineering  Chemistry,  January, 
1915,  page  59,  Mr.  W.  H.  Bowker,  President  of  the  Bowker  Fertilizer 
Company,  a  branch  of  the  American  Agricultural  Chemical  Company, 
makes  the  following  statement : 

' '  There  are  7,000,000  tons  of  fertilizer  consumed  in  this  country  annually,  of 
which  probably  5,000,000  tons  are  what  are  called  complete  fertilizers,  that  is,  con- 
taining the  three  essential  elements  of  plant  nutrition — nitrogen,  phosphorus,  and 
potash. ' ' 

"  It  may  be  added  that  the  most  common  so-called  "complete"  fer- 
tilizer used  in  this  country  contains  about  1%  percent  of  potassium, 
while  the  most  common  corn-belt  soil  contains  about  1%  percent  of 
potassium. 

In  the  same  article,  Mr.  Bowker  quotes  the  following  statement 
concerning  the  German  Kali  Works,  or  Potash  Syndicate: 

"The  Syndicate  is  spending  a  million  dollars  a  year  in  its  world- wide  cam- 
paign, *  *  *  *  and  any  farmer  who  cares  to  study  the  statistics  showing  the  enor- 


4  BULLETIN  No.   182  [May, 

mously  increased  use  of  potash  in  American  fertilizers  during  the  past  decade  must 
be  convinced  that  advertising  pays." 

NORMAL  AND  ABNORMAL  SOILS 

"While  some  abnormal  soils,  such  as  the  peat,  or  muck,  of  certain 
Illinois  swamp  lands  and  the  siliceous  sands  of  the  Atlantic  and  Gulf 
Coastal  Plains,  are  positively  deficient  in  potassium,  the  most  com- 
mon, or  normal,  soils  are  richly  supplied.  (Normal  soils  are  those 
which  bear  relation  in  composition  to  the  average  of  the  earth's  crust 
from  which  they  are  formed.)  Thus  2  million  pounds  of  the  common 
corn-belt  land,  corresponding  to  the  plowed  soil  of  an  acre  to  a  depth  of 
about  6%  inches,  contains  about  35,000  pounds  of  potassium  and  1,200 
pounds  of  phosphorus,  while  2  million  pounds  of  the  earth's  crust  con- 
tains as  an  average  about  50,000  pounds  of  potassium  and  2,200  of 
phosphorus. 

While  phosphorus  is  usually  deficient,  all  must  admit  the  abun- 
dance of  potassium  when  the  total  supply  of  normal  soils  is  considered ; 
but  the  contention  is  usually  made  that  the  potassium  of  the  soil  is 
unavailable,  and,  instead  of  being  advised  to  make  it  available,  the 
farmer  is  urged  to  purchase  soluble  potassium  salts  from  commercial 
agents. 

THE  "INSOLUBLE  RESIDUE" 

The  chief  purpose  of  the  investigations  reported  in  this  bulletin 
was  to  secure  information  as  to  the  power  of  decaying  organic  matter 
to  liberate  potassium  from  the  soil 's  abundant  supply. 

The  method  of  soil  analysis  commonly  employed  by  many  investi- 
gators involves  the  digestion  of  a  certain  amount  (10  grams)  of  the 
soil  with  a  certain  quantity  (100  cubic  centimeters)  of  a  certain  acid 
(hydrochloric)  of  a  certain  strength  (specific  gravity  1.115),  at  a  cer- 
tain temperature  (the  boiling  point  of  water),  for  a  certain  time  (10 
hours).  The  "insoluble  residue"  is  then  discarded  as  valueless,  and 
analyses  are  made  of  only  that  part  of  the  soil  which  dissolves  under 
these  specific  conditions  arbitrarily  fixed. 

As  shown  by  the  many  soil  analyses  reported  in  Bulletin  123,  the 
"acid-soluble"  potassium  found  by  this  method  is,  as  a  general  aver- 
age, only  about  15  to  25  percent  of  the  total  potassium  contained  in 
the  soil.  It  has  seemed  highly  desirable,  therefore,  to  ascertain 
whether  growing  plants  with  their  roots  in  constant  contact  for  months 
with  the  soil  particles  might  not  secure  some  potassium  even  from  this 
"insoluble  residue,"  especially  in  connection  with  the  decomposition 
products  of  organic  matter.  In  other  words,  Can  the  plant  find  value 
in  the  "insoluble  residue"  discarded  by  the  analyst  ? 


19151  POTASSIUM  FROM  THE  SOIL  5 

PREPARATION  OF  POT  CULTURES 

This  investigation  was  begun  in  1909.  The  soil  was  taken  from  the 
Experiment  Station  Farm  of  the  University  of  Illinois,  which  is  rep- 
resentative of  the  brown  silt  loam  of  the  early  Wisconsin  glaciation, 
the  most  common  Illinois  prairie  land.  This  soil  was  found  to  contain 
1.784  percent  of  total  potassium,  corresponding  to  35,680  pounds  in  2 
million  pounds  of  soil,  but  of  this,  an  amount  corresponding  to  28,560 
pounds  remained  in  the  "  insoluble  residue"  by  the  common  method  of 
analysis  just  described. 

By  digesting  many  portions  of  this  soil  with  the  usual  proportion 
and  strength  of  acid  under  regular  conditions  of  temperature  and 
time,  a  sufficient  quantity  of  the  "insoluble  residue"  was  secured  for  a 
series  of  pot  cultures  prepared  as  follows  in  glass  jars  of  about  one- 
gallon  capacity. 

Pots  1  and  la  were  filled  with  normal  soil. 

Pots  2,  3,  and  4  were  filled  with  the  "  insoluble  residue"  from  the 
acid-digestion  of  the  same  kind  of  soil,  and  the  following  chemicals  in 
powdered  form  were  incorporated  with  the  "insoluble  residue": 

Pot  2.     120  grams  of  calcium  carbonate,  CaCO, 

120  grams  of  calcium  phosphate,  Ca,(PO4)2 
30  grams  of  calcium  sulfate,  CaSO4 
5^  grams  of  magnesium  sulfate,  MgSO4 
5  grams  of  iron  chlorid,  Fed, 
Pot  3.     The  same  as  to  Pot  2,  and  in  solution  .160  gram  of  ammonium  ni- 

trate, NH4NO3 

Pot  4.     The  same  as  to  Pot  2,  and  in  solution  .143  gram  of  ammonium  ni- 
trate, NH4NOS,   and  .089  gram   of  sodium  ammonium  phosphate, 


EXPERIMENTS  AND  RESULTS  FOR  1910  AND  1911 

Seven  seeds  of  red  clover  (inoculated)  and  seven  of  rape  were 
planted  in  each  pot.  The  germination  of  the  clover  was  poor,  and  the 
plants  in  the  residue  pots  (2,  3,  and  4)  soon  died.  After  about  two 
weeks  the  rape  plants  were  turned  under. 

To  Pot  2  was  then  added  .200  gram  of  potassium  sulfate,  K2S04, 
and  small  amounts  of  ammonium  nitrate  and  sodium  ammonium  phos- 
phate. Rape  and  clover  seed  were  again  planted  in  all  the  pots  several 
times,  but  in  the  extracted  soil  the  plants  failed  to  live,  even  in  Pot  2, 
and  further  small  applications  of  the  plant-food  solutions  did  not  keep 
them  alive.  The  plants  in  the  normal  soil  were  removed  and  seed 
replanted  whenever  the  other  pots  were  replanted. 

The  extracted  soil  was  so  compact  and  "run  together"  that  the  fail- 
ure was  clearly  due  to  bad  physical  condition.  To  remedy  this,  a  quan- 
tity of  quartz  sand  was  extracted  by  the  usual  method  of  acid-digestion 
described  above,  and  a  sufficient  amount  was  mixed  with  the  extracted 


BULLETIN  No.   182 


[May, 


soil  of  each  pot  to  fill  two  pots.  These  extra  pots  were  numbered  2a, 
3a,  and  4a.  The  soil  of  each  of  these  six  pots  was  then  leached,  or  ex- 
tracted, with  distilled  water,  to  remove  at  least  part  of  the  small 
amounts  of  soluble  potassium,  phosphorus,  and  nitrogen  salts  that 
might  have  remained  from  the  previous  applications. 

Five  clover  seeds,  inoculated  with  clover  bacteria,  were  then  planted 
in  each  pot,  2  of  red  clover  and  3  of  alsike.  With  some  replanting,  an 
irregular  stand  of  alsike  was  finally  secured,  but  the  plants  which  sur- 
vived showed  very  marked  differences,  or  individuality,  in  their  ability 
to  develop.  At  the  close  of  the  season  of  1910  the  highest  yield  of  dry 
matter  from  the  tops  and  roots  of  the  plants  grown  in  the  extracted 
soil  where  no  potassium  fertilizer  had  been  added  (Pot  4a)  amounted 
to  61/2  grams.  From  analyses  of  subsequent  crops  this  dry  matter  was 
estimated  to  contain  221/2  milligrams  of  potassium,  while  the  total  po- 
tassium added  to  this  pot  (in  seed,  inoculation,  and  impurities)  had 
been  only  1.4  milligrams.  The  smallest  yield  was  in  Pot  3a,  in  which 
none  of  the  plants  developed  much  growth.  The  crop  of  i/2  gram  from 
this  pot  was  estimated  to  contain  only  twice  as  much  potassium  as  the 
total  additions. 

Both  tops  and  roots  of  the  1910  crop  were  incorporated  with  the 
extracted  soil  in  the  respective  pots,  and  clover  (chiefly  alsike)  was 
grown  again  in  1911.  A  perfect  stand  was  not  secured,  but  the  plants 
grew  very  much  better  than  in  1910.  The  clover  tops  were  harvested 
on  August  9,  and  again  on  December  21,  when  the  roots  were  also  re- 
moved, air-dried,  and  weighed. 

The  more  important  data  for  the  two  years  are  recorded  in  Table  1. 

TABLE  1. — POTASSIUM  APPLIED  AND  CLOVER  PRODUCED  IN  POT  CULTURES  WITH 
INSOLUBLE  RESIDUE  FROM  SOIL,  1910,  1911 

(Expressed  in  grams) 


Pot 
No. 

Data  for  1910 

Data  for  1911 

Potassium 
applied 

Dry 
produce 

Potassium 
applied 

Air-  dry 
produce 

1 

la 
2 
2a 

.0024 
.0022 
.3155 
.3155 

No  record 
8.0506 
1.7768 
2.8305 

.00005 
.00006 
.00004 
.00007 

32.76 
26.73 
30.33 
18.79 

3 
3a 
4 

4a 

.0013 
.0013 
.0015 
.0014 

1.4146 
.5094 
1.6135 
6.5951 

.00006 
.00010 
.00009 
.00007 

18.88 
13.08 
11.33 
21.29 

Aside  from  Nos.  2  and  2a,  no  pots  received  any  potassium  except 
that  contained  in  the  seed  planted,  in  the  inoculating  material,  and  as 
impurity  in  the  other  plant  foods  used ;  and  to  Pots  2  and  2a  potassium 
fertilizer  was  added  only  for  1910.  The  different  plants  grown  in  1911 


1915} 


POTASSIUM  FROM  THE  SOIL 


also  showed  considerable  individuality,  some  making  much  better 
growth  than  others.  The  tops  and  roots  were  again  incorporated  as 
organic  manure  with  the  extracted  soil  in  the  respective  pots. 

EXPERIMENTS  AND  RESULTS  FOR  1912 

In  1912  two  more  pots  were  added  to  the  series,  Nos.  5  and  5a. 
These  were  filled  with  extracted1  quartz  sand.  The  same  plant-food 
materials  were  added  to  these  pots  as  were  originally  applied  in  pow- 
dered form  to  Pots  2,  3,  and  4,  but  quartz  contains  no  potassium,  and 
thus  differs  from  the  "insoluble  residue"  from  soil  extraction. 

The  entire  series  was  planted  on  February  12, 1912,  with  five  seeds 
of  alsike  to  each  pot ;  and  with  some  replanting  a  good  stand  was  se- 
cured. The  growth  was  fairly  normal  during  the  season.  The  leaves 
were  gathered  from  time  to  time  as  they  seemed  to  mature,  and  all  were 
added  to  the  final  harvest  on  December  12, 1912,  the  produce  from  each 
pot  being  kept  by  itself.  Figs.  1  and  2  show  photographic  views  of 
these  duplicated  series  of  pot  cultures  with  the  1912  clover  crop. 

The  sand  pots,  5  and  5a,  were  planted  and  replanted  several  times, 
and  the  solutions  of  ammonium  nitrate  and  sodium  ammonium  phos- 
phate were  also  added,  but  the  plants  either  remained  very  small  or 
finally  died,  neither  pot  producing  sufficient  crop  to  harvest  or  analyze. 

The  1912  clover  tops  from  all  but  the  sand  pots  were  analyzed,  and 
likewise  the  roots  from  Pots  la,  2a,  3a,  and  4a ;  but  the  roots  from  Pots 
1,  2,  3,  and  4  were  cut  up  and  returned  to  those  respective  pots  before 
planting  the  next  crop.  In  Table  2  are  recorded  the  important  data 
concerning  the  1912  crop. 

TABLE  2. — POTASSIUM  APPLIED,  AIR-DRY  CLOVER  PRODUCED,  AND  POTASSIUM  FOUND 
IN  TOPS  AND  EOOTS  ANALYZED,  1912 

(Expressed  in  grains) 


Pot 
No. 

Potassium 
applied 

Air-dry  produce 

Potassium  found 

Tops 

Roots 

In  tops 

In  roots 

1 
la 
2 
2a 

.00003 
.00003 
.00003 
.00003 

25.56 
34.20 
34.20 
41.07 

14.29 
12.04 
16.30 
26.80 

.2852 
.3310 
.1487 
.1519 

.1109 

.0257 

3 
3a 

4 
4a 

.00004 
.00003 
.00005 
.00007 

34.40 
29.08 
25.25 
38.51 

13.40 
10.47 
13.70 
17.30 

.1406 
.1206 
.0909 
.1324 

.0215 

.0232 

With  no  addition  of  potassium  fertilizer,  the  "insoluble  residue" 
in  Pots  3,  3a,  4,  and  4a  produced  a  larger  average  yield  (31.81  grams) 

Tor  this  extraction  dilute  sulfuric  acid  was  used. 


BULLETIN  No.  182 


FIG.  1. — CLOVER,  1912.  POTS  1  TO  5  FROM  LEFT  TO  RIGHT 

in  1912  than  was  produced  from  the  normal  soil  (29.88  grams,  aver- 
age of  Pots  1  and  la) . 

The  total  potassium  added  to  Pots  3,  3a,  4,  and  4a  in  seed,  inocula- 
tion, and  impurities  amounted  to  6  milligrams  for  the  three  years  1910- 
1912,  while  the  amount  of  potassium  found  in  the  1912  tops  and  roots 
analyzed  from  these  four  pots  was  529.2  milligrams,  of  which  523.2 
milligrams,  or  nearly  99  percent,  must  have  been  secured  from  the 
"insoluble  residue." 

The  pots  used  were  about  6  inches  in  diameter,  and  the  total  area 
of  the  four  pots  was  not  more  than  one  fifty-thousandth  part  of  an 
acre,  so  that  potassium  was  secured  from  the  "insoluble  residue"  at 
the  rate  of  26,160  grams,  or  57  pounds,  per  acre.  The  air-dry  hay  har- 
vested in  1912  from  these  four  pots  amounted  to  127.24  grams,  or  to 
about  7  tons  per  acre,  while  the  yield  from  the  normal  soil  was  about 
6y2  tons  per  acre. 

The  results  indicate  that  after  two  years  of  green  manuring,  suf- 
ficient potassium  was  liberated  from  the  "insoluble  residue"  to  enable 
the  clover  to  be  benefited  by  the  lime  and  phosphate  fertilizers  so  as  to 
outyield  the  crops  on  the  normal  soil,  to  which  no  such  fertilizers  had 
been  applied. 

The  clover  hay  produced  on  the  normal  soil  contained  about  three 
times  as  much  potassium  per  gram  as  was  contained  in  crops  from 
the  "insoluble  residue,"  which  indicates  that  the  actual  requirement 
for  potassium  by  clover  may  be  very  much  less  than  has  been  estimated 
from  the  composition  of  hay  grown  on  ordinary  soils.  In  other  words, 
much  of  the  potassium  commonly  found  in  crops  may  not  be  required 
but  merely  tolerated,  being  taken  up  by  the  growing  plants  because  of 
the  abundance  in  the  soil. 

Thus  the  facts  established  by  these  investigations  tend  to  support 
the  suggestion  made  in  Bulletin  123  (page  216)  and  in  the  Appendix 


1915} 


POTASSIUM  FROM  THE  SOIL 


FIG.  2. — CLOVER,  1912.    POTS  IA  TO  5  A  FROM  LEFT  TO  RIGHT 

of  the  County  Soil  Reports  (under  "The  Potassium  Problem"),  that 
the  benefit  sometimes  produced  by  potash  fertilizers,  when  applied  to 
soils  very  deficient  in  decaying  organic  matter,  may  be  due  in  part  at 
least  to  the  power  of  the  soluble  potash  salt  to  increase  the  availability 
of  phosphorus  or  other  elements. 

EXPERIMENTS  AND  RESULTS  FOR  1913  AND  1914 

As  already  stated,  the  only  organic  matter  incorporated  with  the 
soil  in  preparing  for  the  1913  planting  was  the  clover  roots  in  Pots  1, 
2,  3,  and  4,  and  the  same  plan  was  followed  for  1914 ;  that  is,  the  tops 
from  all  pots  and  the  roots  from  la,  2a,  3a,  and  4a  of  the  1913  and  1914 
crops  were  used  for  analysis,  the  same  as  in  1912.  The  important  data 
for  1913  and  1914  are  combined  in  Table  3. 


TABLE  3. — POTASSIUM  APPLIED,  AIR-DRY  CLOVER  PRODUCED,  AND  POTASSIUM  FOUND 
IN  TOPS  AND  ROOTS  ANALYZED,  1913  AND  1914 

(Expressed  in  grams) 


Pot 
No. 

Potassium 
applied 

Air-  dry  produce 

Potassium  found 

Tops 

Roots 

In  tops 

In  roots 

1 
la 
2 
2a 

.00012 
.00015 
.00016 
.00019 

67.73 
78.29 
51.05 
33.38 

4.81* 
9.821 
15.23 
6.66 

.8758 
.7849 
.1979 
.2526 

.0404 

.0159 

3 

3a 
4 
4a 

.00020 
.00021 
.00012 
.00018 

50.99 
16.84 
76.33 
43.82 

24.80 
3.90 
25.22 
12.72 

.1960 
.1390 
.2909 
.2083 

.0071 

.0158 

*The  roots  in  the  normal  soil  (Pots  1  and  la)  were  found  largely  decayed  at 
harvest  time  and  could  be  only  partially  saved  for  analysis. 


10  BULLETIN  No.   182  [May, 

The  drainage  outlet  from  Pot  3a  became  clogged,  which  probably 
accounts  for  the  very  poor  growth,  but  it  was  apparent  in  both  1913 
and  1914  that  the  clover  grew  better  where  the  roots  from  previous 
crops  were  incorporated  with  the  ' '  insoluble  residue. ' '  However,  the 
authors  have  no  explanation  for  the  larger  amount  of  potassium  recov- 
ered from  Pot  2a  than  from  2,  unless  it  was  the  irregularity  of  subse- 
quent leaching  of  the  potassium  applied  to  those  pots  in  1910.  The 
total  potassium  secured  from  Pots  3,  3a,  4,  and  4a  for  the  two  years 
1913  and  1914  was  857.1  milligrams,  or  428.5  milligrams  per  year,  as 
compared  with  529.2  milligrams  for  1912. 


The  potassium  secured  in  the  clover  tops  from  Pots  3  and  4  was  as 
follows : 

231.5  milligrams  in  1912 

255.3  milligrams  in  1913 

231.6  milligrams  in  1914 

718.4  milligrams  in  three  years 

The  total  amount  of  potassium  added  to  Pots  3  and  4  from  1910  to 
1914  was  3.4  milligrams,  so  that  715  milligrams  were  secured  from  the 
"insoluble  residue"  by  the  clover  tops  analyzed  from  those  two  pots, 
where  the  roots  were  turned  back  each  year.  This  corresponds  to  158 
pounds  of  potassium  per  acre  in  three  years'  crops.  A  100-bushel 
crop  of  corn  contains  about  73  pounds  of  potassium  in  the  grain,  stalks, 
and  cobs. 

It  seems  plainly  evident  that  potassium  need  not  be  purchased  for 
use  on  normal  soils  for  the  production  of  the  staple  farm  crops,  but 
that  it  may  easily  be  liberated  in  abundance  by  means  of  decaying  or- 
ganic matter,  such  as  green  manures,  crop  residues,  and  farm  manures ; 
and,  of  course,  these  materials,  if  applied  in  sufficient  quantity,  will 
supply  nitrogen  and  liberate  phosphorus  from  the  phosphates  natur- 
ally contained  in  the  soil  or  applied  to  it  where  needed. 


/  ; 


"    '    :~2 


UNIVERSITY  OF  ILLINOIS-URBANA 


